Isn't there now a whole other section for discusing stacks and the like? Haven't quite got used to the new set-up yet...

It's impossible to say which stack is the best for someone. In fact, all the stacks have their own characteristics, and so it depends on the tricks you perform.
The Aronson stack contains a lot of cheating routine for example. There are a lot of poker demonstration, a very good any poker hand called for without adjustments… but Mnemonica is higher in coincidence trick, hide in his staystack order…
Someone talk about Si Stebbins, and if you learn by heart a Si Stebbins system you have something similar to Mnemonica : You can take a new deck and convert it in Si Stebbins with only 2 faro instead of 5 with Mnemonica, and naturally you can also start with Si Stebbins to conclude with a new deck order. More, Si Stebbins is also a staystack (after a 26 run), so you have something very similar to Mnemonica with an old stack.

To bring a deck in your favourite order, you can use 2 decks of cards (red and blue) and eliminate the switch. There are strong effects available with 2 decks, and so you can use one of them in mnemonic order, so the other can be freely shuffled. By this way, you can let the spectators shuffling the cards; at the end they don’t remember that they have shuffled only one deck during the act.
There are also several methods to bring a shuffled deck in your memorized order. So if someone gives you a deck, I think that memorized deck art is the most powerful item to perform an impromptu show with cards.
You can also let the spectator shuffled half the deck, perform half stack tricks, and bring the shuffled half in order…

As Nathan say, there are a lot of effects that aren’t stack dependant, but the best stack for you depends on your own magic card tricks. If you want to add the climax of a new deck order, you should be interesting by Mnemonica (or Lennart Green). If you search for great cheating demonstration shows, I think Aronson is higher than Mnemonica, because the routines can be done directly without adjusting your deck (I'm thinking about "Any poker hand called for" and the "Vernon poker demonstration").
It’s important to make the good choice for your stack, because it’s hard to memorize another stack when you know one. Fred Kaps has memorized a 32 cards deck, and when the 52 cards deck became more popular, he abort memorized deck card magic… Good luck